Systems which coherently sum a series of tapped delay lines are relevant to a wide range of signal processing applications, transverse filtering being a prominent example. Another example is that of decoy repeaters. An object will modify any signal reflected from it according to the object's shape, and the object's velocity relative to the signal. This permits a hostile interrogator to identify the nature of such objects, which, if the objects are military platforms such as warships or aircraft, is not desirable. One solution has been to artificially synthesize fake characteristic echo signatures in response to receipt of an interrogating signal. Thus, for example, a series of decoy buoys deployed at sea could simulate the presence of a naval flotilla, and thereby potentially disrupt enemy plans.
FIG. 1 illustrates broadly how this is done for a ship 5 and an aircraft 3, in the line of sight of an interrogating signal 2. For illustrative purposes, signal 2 can be a radar pulse, but could as well be any linear signal pulse, of which sonar or acoustic signals are other examples. As signal 2 hits aircraft 3 and ship 5, it bounces off of their major reflective surfaces, which, for ship 5 are the hull 4, superstructure 6, and smokestack 8, and for the aircraft are the nose 9 and the wings 7. The echo from craft 3, 5 will be the superposition of the echoes from surfaces 4, 6, 8, and 9, 7, and because these surfaces are at different places along the line of sight of signal 2, the superimposed reflections will be out of phase with one another by the differing times of flight of signal 2 to each reflecting surface. This tends to lengthen the echo by an amount equal to the round trip time of flight of signal 2 between the nearest and farthest major reflector, in the case of ship 5 hull 4 and smoke stack 8, and to make the echo of varying magnitude as dictated by the varying radar cross sections of the reflecting surfaces. Furthermore, movement of aircraft 3 or ship 5 relative to signal 2 will Doppler shift the returned echos. Thus any platform which reflects signal will in effect frequency modulate signal 2, such that the returned echoes permit an interrogator to infer the nature and motion of the platform. The most common way to detect a Doppler shift is, responsive to a series of interrogation pulses, compare echoes from consecutive pulses. Thus, an imaging interrogator, such as a search radar, SAR, ISAR, etc., would infer Doppler by comparing consecutive echoes, and do so on a range bin by range bin basis. The interrogator can infer strength of reflection, e.g. radar cross-section, in a range bin from only one echo returned from the range bin by checking echo strength, although as a matter of prudence, it would likely check several such echoes to ensure that echo strength doesn't vary greatly.
Any credible repeater decoy must simulate the temporal lengthening and amplitude modulation caused by plural, recessed, reflective surfaces, and a simulate a realistic Doppler shift for each surface.
Conventionally this is done by analog systems which receive an interrogating signal and pass it through a length of cable having serial taps along its length, one tap per range bin (also called range cell, or downrange range cell). Each tap modulates the signal in amplitude and/or frequency to simulate reflection from the reflective surfaces within that range bin. Total path length of signals traversing the respective taps are selected to correspond to the differing times of flight of the interrogating signal to the respective range bins. Finally, the signals from the taps are summed, and the signal thus synthesized is retransmitted. In this manner, the system returns what appears to be an echo from an object located within the selected range bins, and having a signature indicative of the object to be simulated, e.g. a ship or aircraft in motion.
Unfortunately, analog systems have drawbacks which limit their usefulness as decoys. They are inherently noisy, and can hold an incoming signal only a short time for processing before the signal deteriorates below noise. This limits system bandwidth, and permits effective simulation of only small objects. Further, analog systems are costly and very bulky, the latter being a particular concern for military platforms, where space is extremely limited. Finally, analog systems cannot readily change operating parameters such as relative delays among taps, or the amount of modulation in the various taps. This means that analog repeaters cannot switch among different simulated objects on the fly, but rather must typically be fabricated for one specific target.